416 WRITINGS OF JOSEPH HENRY. [1856 



of an increase of temperature were observed. A more deli- 

 cate method of making the experiment next suggested itself. 

 The tube containing the drop of red ink, with its cork, was 

 removed, and the point of a compound wire formed of copper 

 and iron was thrust into the substance of the rubber, while 

 the other ends of the wire were connected with a delicate 

 galvanometer. The needle was suffered to come to rest, the 

 tuning-fork was then vibrated, and its impulses transmitted 

 to the rubber. A very perceptible increase of temperature 

 was the result. The needle moved through an arc of from 

 one to two and a half degrees. The experiment was varied 

 and many times repeated; the motions of the needle were 

 always in the same direction, namely, in that which was 

 produced when the point of the compound wire was heated 

 by momentary contact with the fingers. The amount of heat 

 generated in this way however is small, and indeed in all 

 cases in which it is generated by mechanical means the 

 amount evolved appears very small in comparison with the 

 labor expended in producing it. Joule has shown that the 

 mechanical energy generated in a pound weight by falling 

 through a space of seven hundred and fifty feet elevates the 

 temperature of a pound of water one degree. 



It is evident that an object like India-rubber actually 

 destroys a portion of the sound, and hence in cases in which 

 entire non-conduction is required this substance can prob- 

 ably be employed with perfect success. 



The tuning-fork was next pressed upon a solid brick wall, 

 and the duration of the vibration from a number of trials 

 was eighty-eight seconds. Against a wall of lath and plaster 

 the sound was louder and continued only eighteen seconds. 



From these experiments we may infer that if a room were 

 lined with a wainscot of thin boards and a space left between 

 the wall and the wood, the loudness of the echo of a single 

 noise would be increased while the duration of the reson- 

 ance would be diminished. If however the thin board were 

 glued or cemented in solid connection to the wall, or em- 

 bedded in the mortar, then the effect would be a feeble echo 

 and a long continued resonance, similar to that from the 



